Journal tiering in a continuous data protection system using deduplication-based storage

ABSTRACT

In one aspect, a method includes checking periodically, using a processor, for a value associated with data in a data block in a higher tier journal stored on a first storage array having deduplication-based functionality and copying the data in the data block from the higher tier journal to a lower tier journal in a second storage array if the data in the data block exists only in an UNDO stream in the higher tier journal. The first storage array and the second storage array are used in a continuous data protection system to replicate a volume. The method also includes replacing the data in the data block in the journal with a zero block if the data in the data block exists only in the UNDO stream.

BACKGROUND

Computer data is vital to today's organizations and a significant partof protection against disasters is focused on data protection. Assolid-state memory has advanced to the point where cost of memory hasbecome a relatively insignificant factor, organizations can afford tooperate with systems that store and process terabytes of data.

Conventional data protection systems include tape backup drives, forstoring organizational production site data on a periodic basis. Anotherconventional data protection system uses data replication, by creating acopy of production site data of an organization on a secondary backupstorage system, and updating the backup with changes. The backup storagesystem may be situated in the same physical location as the productionstorage system, or in a physically remote location. Data replicationsystems generally operate either at the application level, at the filesystem level, or at the data block level.

SUMMARY

In one aspect, a method includes checking periodically, using aprocessor, for a value associated with data in a data block in a highertier journal stored on a first storage array having deduplication-basedfunctionality and copying the data in the data block from the highertier journal to a lower tier journal in a second storage array if thedata in the data block exists only in an UNDO stream in the higher tierjournal. The first storage array and the second storage array are usedin a continuous data protection system to replicate a volume. The methodalso includes replacing the data in the data block in the journal with azero block if the data in the data block exists only in the UNDO stream.

In another aspect, an apparatus includes electronic hardware circuitryconfigured to check periodically for a value associated with data in adata block in a higher tier journal stored on a first storage arrayhaving deduplication-based functionality, copy the data in the datablock from the higher tier journal to a lower tier journal in a secondstorage array if the data in the data block exists only in an UNDOstream in the higher tier journal and replace the data in the data blockin the journal with a zero block if the data in the data block existsonly in the UNDO stream. The first storage array and the second storagearray are used in a continuous data protection system to replicate avolume.

In a further aspect, an article includes a non-transitorycomputer-readable medium that stores computer-executable instructions.The instructions cause a machine to check periodically for a valueassociated with data in a data block in a higher tier journal stored ona first storage array having deduplication-based functionality, copy thedata in the data block from the higher tier journal to a lower tierjournal in a second storage array if the data in the data block existsonly in an UNDO stream in the higher tier journal and replace the datain the data block in the journal with a zero block if the data in thedata block exists only in the UNDO stream. The first storage array andthe second storage array are used in a continuous data protection systemto replicate a volume.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example of a data protection system.

FIG. 2 is an illustration of an example of a journal history of writetransactions for a storage system.

FIG. 3 is a block diagram of another example of a replication systemusing deduplication based storage.

FIG. 4 is a flowchart of an example of a process to form journaltiering.

FIG. 5 is a flowchart of an example of a process to use journal tiering.

FIG. 6 is a computer on which any of the processes of FIGS. 4 and 5 maybe implemented.

DETAILED DESCRIPTION

Described herein is an approach to use journal tiering in a continuousdata protection system using deduplication-based storage volumes.Storing a journal using a flash storage is relatively expensive.However, by using journal tiering the amount of flash memory used isreduced, because the journal is kept partially on a flash baseddeduplication storage and partially on a non-flash based standardvolume.

The following definitions may be useful in understanding thespecification and claims.

BACKUP SITE—a facility where replicated production site data is stored;the backup site may be located in a remote site or at the same locationas the production site;

BOOKMARK—a bookmark is metadata information stored in a replicationjournal which indicates a point in time.

DATA PROTECTION APPLIANCE (DPA)—a computer or a cluster of computersresponsible for data protection services including inter alia datareplication of a storage system, and journaling of I/O requests issuedby a host computer to the storage system;

HOST—at least one computer or networks of computers that runs at leastone data processing application that issues I/O requests to one or morestorage systems; a host is an initiator with a SAN;

HOST DEVICE—an internal interface in a host, to a logical storage unit;

IMAGE—a copy of a logical storage unit at a specific point in time;

INITIATOR—a node in a SAN that issues I/O requests;

I/O REQUEST—an input/output request (sometimes referred to as an I/O),which may be a read I/O request (sometimes referred to as a read requestor a read) or a write I/O request (sometimes referred to as a writerequest or a write);

JOURNAL—a record of write transactions issued to a storage system; usedto maintain a duplicate storage system, and to roll back the duplicatestorage system to a previous point in time;

LOGICAL UNIT—a logical entity provided by a storage system for accessingdata from the storage system. The logical disk may be a physical logicalunit or a virtual logical unit;

LUN—a logical unit number for identifying a logical unit;

PHYSICAL LOGICAL UNIT—a physical entity, such as a disk or an array ofdisks, for storing data in storage locations that can be accessed byaddress;

PRODUCTION SITE—a facility where one or more host computers run dataprocessing applications that write data to a storage system and readdata from the storage system;

REMOTE ACKNOWLEDGEMENTS—an acknowledgement from remote DPA to the localDPA that data arrived at the remote DPA (either to the appliance or thejournal)

SPLITTER ACKNOWLEDGEMENT—an acknowledgement from a DPA to the protectionagent (splitter) that data has been received at the DPA; this may beachieved by an SCSI status command.

SAN—a storage area network of nodes that send and receive an I/O andother requests, each node in the network being an initiator or a target,or both an initiator and a target;

SOURCE SIDE—a transmitter of data within a data replication workflow,during normal operation a production site is the source side; and duringdata recovery a backup site is the source side, sometimes called aprimary side;

STORAGE SYSTEM—a SAN entity that provides multiple logical units foraccess by multiple SAN initiators

TARGET—a node in a SAN that replies to I/O requests;

TARGET SIDE—a receiver of data within a data replication workflow;during normal operation a back site is the target side, and during datarecovery a production site is the target side, sometimes called asecondary side;

VIRTUAL LOGICAL UNIT—a virtual storage entity which is treated as alogical unit by virtual machines;

WAN—a wide area network that connects local networks and enables them tocommunicate with one another, such as the Internet.

A description of journaling and some techniques associated withjournaling may be described in the patent titled “METHODS AND APPARATUSFOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION” and with U.S.Pat. No. 7,516,287, which is hereby incorporated by reference.

Before describing a replication system that includes a deduplicationbased storage volume, an example replication system is first describedin FIGS. 1 and 2.

An Example of a Replication System

Referring to FIG. 1, a data protection system 100 includes two sites;Site I, which is a production site, and Site II, which is a backup siteor replica site. Under normal operation the production site is thesource side of system 100, and the backup site is the target side of thesystem. The backup site is responsible for replicating production sitedata. Additionally, the backup site enables roll back of Site I data toan earlier pointing time, which may be used in the event of datacorruption of a disaster, or alternatively in order to view or to accessdata from an earlier point in time.

FIG. 1 is an overview of a system for data replication of eitherphysical or virtual logical units. Thus, one of ordinary skill in theart would appreciate that in a virtual environment a hypervisor, in oneexample, would consume logical units and generate a distributed filesystem on them such as VMFS creates files in the file system and exposethe files as logical units to the virtual machines (each VMDK is seen asa SCSI device by virtual hosts). In another example, the hypervisorconsumes a network based file system and exposes files in the NFS asSCSI devices to virtual hosts.

During normal operations, the direction of replicate data flow goes fromsource side to target side. It is possible, however, for a user toreverse the direction of replicate data flow, in which case Site Istarts to behave as a target backup site, and Site II starts to behaveas a source production site. Such change of replication direction isreferred to as a “failover”. A failover may be performed in the event ofa disaster at the production site, or for other reasons. In some dataarchitectures, Site I or Site II behaves as a production site for aportion of stored data, and behaves simultaneously as a backup site foranother portion of stored data. In some data architectures, a portion ofstored data is replicated to a backup site, and another portion is not.

The production site and the backup site may be remote from one another,or they may both be situated at a common site, local to one another.Local data protection has the advantage of minimizing data lag betweentarget and source, and remote data protection has the advantage is beingrobust in the event that a disaster occurs at the source side.

The source and target sides communicate via a wide area network (WAN)128, although other types of networks may be used.

Each side of system 100 includes three major components coupled via astorage area network (SAN); namely, (i) a storage system, (ii) a hostcomputer, and (iii) a data protection appliance (DPA). Specifically withreference to FIG. 1, the source side SAN includes a source host computer104, a source storage system 108, and a source DPA 112. Similarly, thetarget side SAN includes a target host computer 116, a target storagesystem 120, and a target DPA 124. As well, the protection agent(sometimes referred to as a splitter) may run on the host, or on thestorage, or in the network or at a hypervisor level, and that DPAs areoptional and DPA code may run on the storage array too, or the DPA 124may run as a virtual machine.

Generally, a SAN includes one or more devices, referred to as “nodes”.Anode in a SAN may be an “initiator” or a “target”, or both. Aninitiator node is a device that is able to initiate requests to one ormore other devices; and a target node is a device that is able to replyto requests, such as SCSI commands, sent by an initiator node. A SAN mayalso include network switches, such as fiber channel switches. Thecommunication links between each host computer and its correspondingstorage system may be any appropriate medium suitable for data transfer,such as fiber communication channel links.

The host communicates with its corresponding storage system using smallcomputer system interface (SCSI) commands.

System 100 includes source storage system 108 and target storage system120. Each storage system includes physical storage units for storingdata, such as disks or arrays of disks. Typically, storage systems 108and 120 are target nodes. In order to enable initiators to send requeststo storage system 108, storage system 108 exposes one or more logicalunits (LU) to which commands are issued. Thus, storage systems 108 and120 are SAN entities that provide multiple logical units for access bymultiple SAN initiators.

Logical units are a logical entity provided by a storage system, foraccessing data stored in the storage system. The logical unit may be aphysical logical unit or a virtual logical unit. A logical unit isidentified by a unique logical unit number (LUN). Storage system 108exposes a logical unit 136, designated as LU A, and storage system 120exposes a logical unit 156, designated as LU B.

LU B is used for replicating LU A. As such, LU B is generated as a copyof LU A. In one embodiment, LU B is configured so that its size isidentical to the size of LU A. Thus, for LU A, storage system 120 servesas a backup for source side storage system 108. Alternatively, asmentioned hereinabove, some logical units of storage system 120 may beused to back up logical units of storage system 108, and other logicalunits of storage system 120 may be used for other purposes. Moreover,there is symmetric replication whereby some logical units of storagesystem 108 are used for replicating logical units of storage system 120,and other logical units of storage system 120 are used for replicatingother logical units of storage system 108.

System 100 includes a source side host computer 104 and a target sidehost computer 116. A host computer may be one computer, or a pluralityof computers, or a network of distributed computers, each computer mayinclude inter alia a conventional CPU, volatile and non-volatile memory,a data bus, an I/O interface, a display interface and a networkinterface. Generally a host computer runs at least one data processingapplication, such as a database application and an e-mail server.

Generally, an operating system of a host computer creates a host devicefor each logical unit exposed by a storage system in the host computerSAN. A host device is a logical entity in a host computer, through whicha host computer may access a logical unit. Host device 104 identifies LUA and generates a corresponding host device 140, designated as Device A,through which it can access LU A. Similarly, host computer 116identifies LU B and generates a corresponding device 160, designated asDevice B.

In the course of continuous operation, host computer 104 is a SANinitiator that issues I/O requests (write/read operations) through hostdevice 140 to LU A using, for example, SCSI commands. Such requests aregenerally transmitted to LU A with an address that includes a specificdevice identifier, an offset within the device, and a data size. Offsetsare generally aligned to 512 byte blocks. The average size of a writeoperation issued by host computer 104 may be, for example, 10 kilobytes(KB); i.e., 20 blocks. For an I/O rate of 50 megabytes (MB) per second,this corresponds to approximately 5,000 write transactions per second.

System 100 includes two data protection appliances, a source side DPA112 and a target side DPA 124. A DPA performs various data protectionservices, such as data replication of a storage system, and journalingof I/O requests issued by a host computer to source side storage systemdata. As explained in detail herein, when acting as a target side DPA, aDPA may also enable roll back of data to an earlier point in time, andprocessing of rolled back data at the target site. Each DPA 112 and 124is a computer that includes inter alia one or more conventional CPUs andinternal memory.

For additional safety precaution, each DPA is a cluster of suchcomputers. Use of a cluster ensures that if a DPA computer is down, thenthe DPA functionality switches over to another computer. The DPAcomputers within a DPA cluster communicate with one another using atleast one communication link suitable for data transfer via fiberchannel or IP based protocols, or such other transfer protocol. Onecomputer from the DPA cluster serves as the DPA leader. The DPA clusterleader coordinates between the computers in the cluster, and may alsoperform other tasks that require coordination between the computers,such as load balancing.

In the architecture illustrated in FIG. 1, DPA 112 and DPA 124 arestandalone devices integrated within a SAN. Alternatively, each of DPA112 and DPA 124 may be integrated into storage system 108 and storagesystem 120, respectively, or integrated into host computer 104 and hostcomputer 116, respectively. Both DPAs communicate with their respectivehost computers through communication lines such as fiber channels using,for example, SCSI commands or any other protocol.

DPAs 112 and 124 are configured to act as initiators in the SAN; i.e.,they can issue I/O requests using, for example, SCSI commands, to accesslogical units on their respective storage systems. DPA 112 and DPA 124are also configured with the necessary functionality to act as targets;i.e., to reply to I/O requests, such as SCSI commands, issued by otherinitiators in the SAN, including inter alia their respective hostcomputers 104 and 116. Being target nodes, DPA 112 and DPA 124 maydynamically expose or remove one or more logical units.

As described hereinabove, Site I and Site II may each behavesimultaneously as a production site and a backup site for differentlogical units. As such, DPA 112 and DPA 124 may each behave as a sourceDPA for some logical units, and as a target DPA for other logical units,at the same time.

Host computer 104 and host computer 116 include protection agents 144and 164, respectively. Protection agents 144 and 164 intercept SCSIcommands issued by their respective host computers, via host devices tological units that are accessible to the host computers. A dataprotection agent may act on an intercepted SCSI commands issued to alogical unit, in one of the following ways: send the SCSI commands toits intended logical unit; redirect the SCSI command to another logicalunit; split the SCSI command by sending it first to the respective DPA;after the DPA returns an acknowledgement, send the SCSI command to itsintended logical unit; fail a SCSI command by returning an error returncode; and delay a SCSI command by not returning an acknowledgement tothe respective host computer.

A protection agent may handle different SCSI commands, differently,according to the type of the command. For example, a SCSI commandinquiring about the size of a certain logical unit may be sent directlyto that logical unit, while a SCSI write command may be split and sentfirst to a DPA associated with the agent. A protection agent may alsochange its behavior for handling SCSI commands, for example as a resultof an instruction received from the DPA.

Specifically, the behavior of a protection agent for a certain hostdevice generally corresponds to the behavior of its associated DPA withrespect to the logical unit of the host device. When a DPA behaves as asource site DPA for a certain logical unit, then during normal course ofoperation, the associated protection agent splits I/O requests issued bya host computer to the host device corresponding to that logical unit.Similarly, when a DPA behaves as a target device for a certain logicalunit, then during normal course of operation, the associated protectionagent fails I/O requests issued by host computer to the host devicecorresponding to that logical unit.

Communication between protection agents and their respective DPAs mayuse any protocol suitable for data transfer within a SAN, such as fiberchannel, or SCSI over fiber channel. The communication may be direct, orvia a logical unit exposed by the DPA. Protection agents communicatewith their respective DPAs by sending SCSI commands over fiber channel.

Protection agents 144 and 164 are drivers located in their respectivehost computers 104 and 116. Alternatively, a protection agent may alsobe located in a fiber channel switch, or in any other device situated ina data path between a host computer and a storage system or on thestorage system itself. In a virtualized environment, the protectionagent may run at the hypervisor layer or in a virtual machine providinga virtualization layer.

What follows is a detailed description of system behavior under normalproduction mode, and under recovery mode.

In production mode DPA 112 acts as a source site DPA for LU A. Thus,protection agent 144 is configured to act as a source side protectionagent; i.e., as a splitter for host device A. Specifically, protectionagent 144 replicates SCSI I/O write requests. A replicated SCSI I/Owrite request is sent to DPA 112. After receiving an acknowledgementfrom DPA 124, protection agent 144 then sends the SCSI I/O write requestto LU A. After receiving a second acknowledgement from storage system108 host computer 104 acknowledges that an I/O command complete.

When DPA 112 receives a replicated SCSI write request from dataprotection agent 144, DPA 112 transmits certain I/O informationcharacterizing the write request, packaged as a “write transaction”,over WAN 128 to DPA 124 on the target side, for journaling and forincorporation within target storage system 120.

DPA 112 may send its write transactions to DPA 124 using a variety ofmodes of transmission, including inter alia (i) a synchronous mode, (ii)an asynchronous mode, and (iii) a snapshot mode. In synchronous mode,DPA 112 sends each write transaction to DPA 124, receives back anacknowledgement from DPA 124, and in turns sends an acknowledgement backto protection agent 144. Protection agent 144 waits until receipt ofsuch acknowledgement before sending the SCSI write request to LU A.

In asynchronous mode, DPA 112 sends an acknowledgement to protectionagent 144 upon receipt of each I/O request, before receiving anacknowledgement back from DPA 124.

In snapshot mode, DPA 112 receives several I/O requests and combinesthem into an aggregate “snapshot” of all write activity performed in themultiple I/O requests, and sends the snapshot to DPA 124, for journalingand for incorporation in target storage system 120. In snapshot mode DPA112 also sends an acknowledgement to protection agent 144 upon receiptof each I/O request, before receiving an acknowledgement back from DPA124.

For the sake of clarity, the ensuing discussion assumes that informationis transmitted at write-by-write granularity.

While in production mode, DPA 124 receives replicated data of LU A fromDPA 112, and performs journaling and writing to storage system 120. Whenapplying write operations to storage system 120, DPA 124 acts as aninitiator, and sends SCSI commands to LU B.

During a recovery mode, DPA 124 undoes the write transactions in thejournal, so as to restore storage system 120 to the state it was at, atan earlier time.

As described hereinabove, LU B is used as a backup of LU A. As such,during normal production mode, while data written to LU A by hostcomputer 104 is replicated from LU A to LU B, host computer 116 shouldnot be sending I/O requests to LU B. To prevent such I/O requests frombeing sent, protection agent 164 acts as a target site protection agentfor host Device B and fails I/O requests sent from host computer 116 toLU B through host Device B.

Target storage system 120 exposes a logical unit 176, referred to as a“journal LU”, for maintaining a history of write transactions made to LUB, referred to as a “journal”. Alternatively, journal LU 176 may bestriped over several logical units, or may reside within all of or aportion of another logical unit. DPA 124 includes a journal processor180 for managing the journal.

Journal processor 180 functions generally to manage the journal entriesof LU B. Specifically, journal processor 180 enters write transactionsreceived by DPA 124 from DPA 112 into the journal, by writing them intothe journal LU, reads the undo information for the transaction from LUB. updates the journal entries in the journal LU with undo information,applies the journal transactions to LU B, and removes already-appliedtransactions from the journal.

Referring to FIG. 2, which is an illustration of a write transaction 200for a journal. The journal may be used to provide an adaptor for accessto storage 120 at the state it was in at any specified point in time.Since the journal contains the “undo” information necessary to roll backstorage system 120, data that was stored in specific memory locations atthe specified point in time may be obtained by undoing writetransactions that occurred subsequent to such point in time.

Write transaction 200 generally includes the following fields: one ormore identifiers; a time stamp, which is the date & time at which thetransaction was received by source side DPA 112; a write size, which isthe size of the data block; a location in journal LU 176 where the datais entered; a location in LU B where the data is to be written; and thedata itself.

Write transaction 200 is transmitted from source side DPA 112 to targetside DPA 124. As shown in FIG. 2, DPA 124 records the write transaction200 in the journal that includes four streams. A first stream, referredto as a DO stream, includes new data for writing in LU B. A secondstream, referred to as an DO METADATA stream, includes metadata for thewrite transaction, such as an identifier, a date & time, a write size, abeginning address in LU B for writing the new data in, and a pointer tothe offset in the DO stream where the corresponding data is located.Similarly, a third stream, referred to as an UNDO stream, includes olddata that was overwritten in LU B; and a fourth stream, referred to asan UNDO METADATA, include an identifier, a date & time, a write size, abeginning address in LU B where data was to be overwritten, and apointer to the offset in the UNDO stream where the corresponding olddata is located.

In practice each of the four streams holds a plurality of writetransaction data. As write transactions are received dynamically bytarget DPA 124, they are recorded at the end of the DO stream and theend of the DO METADATA stream, prior to committing the transaction.During transaction application, when the various write transactions areapplied to LU B, prior to writing the new DO data into addresses withinthe storage system, the older data currently located in such addressesis recorded into the UNDO stream. In some examples, the metadata stream(e.g., UNDO METADATA stream or the DO METADATA stream) and the datastream (e.g., UNDO stream or DO stream) may be kept in a single streameach (i.e., one UNDO data and UNDO METADATA stream and one DO data andDO METADATA stream) by interleaving the metadata into the data stream.

Referring to FIG. 3, a replication system 300 includes a DPA 306 a, afirst storage array 308 a and a second storage array 308 b at theproduction site and a DPA 306 b, a first storage array 338 a and asecond storage array 338 b at the production site.

The first storage array 308 a includes a data protection agent 320, ajournal volume 322 a and a user volume 324. The second storage array 308b includes a journal volume 326 b. The journal volume 322 a includes afirst part of a journal, journal 326 a and the journal volume 322 bincludes a second part of the journal 326 b.

The first storage array 338 a includes a data protection agent 350, ajournal volume 352 a and a replicated user volume 324. The secondstorage array 338 b includes a journal volume 356 b. The journal volume352 a includes a first part of a journal, journal 356 a and the journalvolume 352 b includes a second part of the journal 356 b.

As will be explained herein, parts of a journal are divided betweendifferent storage arrays called herein journal tiering. For example, thefirst part of the journal 326 a, called a higher tier journal, is storedin the first storage array 308 a and the second part of the journal 326b, called a lower tier journal, is stored in the second storage 308 b.In another example, the first part of the journal 356 a, called a highertier journal, is stored in the first storage array 338 a and the secondpart of the journal 356 b, called a lower tier journal, is stored in thesecond storage 338 b.

In one example, the second storage array 308 b and the second storagearray 338 b may be for example an EMC® VNX, which can contain SATA, FCand also some flash drives, but is not a full flash-based array.

The first storage array 308 a and first storage array 338 a includededuplication-based storage volumes. Deduplication-based storage is astorage paradigm that utilizes deduplication technology at the very coreof the storage system. For example, I/Os arriving to thededuplication-based storage volume are divided into fixed chunks of data(e.g., 4K). A cryptographically strong signature or fingerprint iscalculated for each data chunk. The uniqueness of this signature is thekey factor of this kind of storage so that a data chunk is uniquelyidentified by its data signature. The deduplication-based storagevolumes are sequences of the signature data, with the actual data storedseparately. Or viewed in another way, the storage data is a bigpersistent hash table with the signatures used as keys, and the volumesare sequences of those keys.

Thus, actual data may be stored only once in a system while othercomponents in the system reference the data by its signature. Thisreference relates not only to the actual volume storage of the data, butmay also relate to memory caches, snapshots, internal copying of dataand so forth. The actual data blocks are not copied but rather theirsignature is or even just pointers to signatures may be copied. Areference count is kept on each data block, which counts the number ofreferences to that block. When the block is no longer referenced it maybe removed from the storage.

In one example, the first storage arrays 308 a, 338 b are flash drives.In order to improve efficiency, the data lookup by signature should bevery fast. Since the data lookup is a random access operation by natureit is beneficial that flash technology be used. Deduplication-basedstorage volumes utilize the high I/O operations per second (IOPS) andrandom access nature of flash technology. The deduplication complementsflash technology limitations by providing a compact representation ofthe data, and efficiently utilizing the relatively expensive flashcapacity. The combination of flash drives and deduplication basedstorage in the front end forms a very high performance system.

A flash-based storage array is expensive and has limited amount ofstorage. While it is desired to maintain the journal at thededuplication-based storage array, this might be too expensive. Tieringof parts of the journal which actually consume space in thededuplication-based storage (i.e., blocks which are not shared), cansignificantly reduce the amount of space used by the deduplication-basedstorage

Referring to FIG. 4, an example of a process to form journal tiering isa process 400. The process 400 checks periodically a value for eachjournal data block (410). For example, the DPA 306 b checks periodicallya reference count for each data block in the journal 356 a stored on thefirst storage array 338 a.

Process 400 determines if the data in the data block exists only in anUNDO stream (422). In some examples, blocks may be shared by multipleUNDO streams in which case the system may also choose to move blocks toa lower tier journal. In some examples, erasing the data in the datablock will actually save space in the storage. The DPA 306 b, forexample, determines if the data in the data block exists only in an UNDOstream by checking the reference count. For example, data in the datablock exists in an UNDO stream and the current reference count for thisdata is 1. This means that erasing the data from the UNDO stream, willallow freeing of the data block.

If the data in the data block exists only in the UNDO stream, process400 determines if the data in the data block meets a predetermined value(430). For example, the DPA 306 b determines if the data in the datablock is older than a predetermined amount of time. In another example,the DPA 306 b determines if the data consumed by the journal is largerthan a predetermined amount of data.

If the data in the data block is older than a predetermined amount oftime (e.g., older than one minute), process 400 copies the data in thedata block from the higher tier journal to a lower tier journal (438).For example, the DPA 306 b copies the data in the data block from thehigher tier journal 356 a in the first storage array 338 to the lowertier journal 356 b in the second storage array 338 b.

Process 400 replaces the data in the data block in the journal with azero block (442) and marks the data block in the metadata to point tothe lower tier journal (450). A zero block may be a block of zero datafor which the deduplication-based storage consumes no space. Forexample, the DPA 306 b replaces the data in the data block in thejournal with a zero block (e.g., by using a SCSI punch (SCSI unmap)command), and marks the data block in the metadata in the UNDO METADATAstream to point to the lower tier journal.

Referring to FIG. 5, an example of a process to use journal tiering is aprocess 500. In one example, process 500 may be used when the system 300tries to roll to an earlier point in time.

Process 500 reads metadata from an UNDO METADATA stream (502). Forexample, DPA 306 b reads metadata from UNDO METADATA stream in thejournal 356 a.

Process 500 determines if the metadata points to data is in a lower tierjournal (504). If the metadata points to the lower tier storage, process500 reads data from a lower tier journal (506). For example, the DPA 306a reads data from the lower tier journal 356 b in the second storagearray 338 b.

Process 500 copies data from the replicated volume to a redo log (i.e.,to the DO stream, and metadata is updated in the DO METADATA stream)(508). For example, DPA 306 b copies data from the replicated uservolume 354 to the redo log in the journal 356 a. Data may be copied byusing a vendor specific read signature command to read currentsignatures in the replica volume and then using a vendor specific writesignature command to write the data with the new signatures to the DOstream, or using a SCSI xcopy command.

A read signature command is a vendor specific SCSI command which gets anoffset and a number of blocks, and returns the hash signatures of theblocks, where a block can be the basic block of deduplication, forexample, of 4 KB.

A write signature command is a command which gets an offset and asignature, and if the deduplication storage contains data matching thesignature, data is written to the offset indicated in the command.Otherwise a special status indicating NOT_EXIST is returned.

Process 500 applies data to replicated volume (514). For example, theDPA 306 b writes the data read from the UNDO stream from the lower tierjournal 356 b to the replicated user volume 354. Data from the UNDOstream which is in the higher tier journal 356 a may be copied by theDPA 306 b to the replicated user volume 354 using either the xcopycommand or the read signature/write signature commands.

Referring to FIG. 6, in one example, a computer 600 includes a processor602, a volatile memory 604, a non-volatile memory 606 (e.g., hard disk)and the user interface (UI) 608 (e.g., a graphical user interface, amouse, a keyboard, a display, touch screen and so forth). Thenon-volatile memory 606 stores computer instructions 612, an operatingsystem 616 and data 618. In one example, the computer instructions 612are executed by the processor 602 out of volatile memory 604 to performall or part of the processes described herein (e.g., processes 400 and500).

The processes described herein (e.g., processes 400 and 500) are notlimited to use with the hardware and software of FIG. 6; they may findapplicability in any computing or processing environment and with anytype of machine or set of machines that is capable of running a computerprogram. The processes described herein may be implemented in hardware,software, or a combination of the two. The processes described hereinmay be implemented in computer programs executed on programmablecomputers/machines that each includes a processor, a non-transitorymachine-readable medium or other article of manufacture that is readableby the processor (including volatile and non-volatile memory and/orstorage elements), at least one input device, and one or more outputdevices. Program code may be applied to data entered using an inputdevice to perform any of the processes described herein and to generateoutput information.

The system may be implemented, at least in part, via a computer programproduct, (e.g., in a non-transitory machine-readable storage medium suchas, for example, a non-transitory computer-readable medium), forexecution by, or to control the operation of, data processing apparatus(e.g., a programmable processor, a computer, or multiple computers)).Each such program may be implemented in a high level procedural orobject-oriented programming language to communicate with a computersystem. However, the programs may be implemented in assembly or machinelanguage. The language may be a compiled or an interpreted language andit may be deployed in any form, including as a stand-alone program or asa module, component, subroutine, or other unit suitable for use in acomputing environment. A computer program may be deployed to be executedon one computer or on multiple computers at one site or distributedacross multiple sites and interconnected by a communication network. Acomputer program may be stored on a non-transitory machine-readablemedium that is readable by a general or special purpose programmablecomputer for configuring and operating the computer when thenon-transitory machine-readable medium is read by the computer toperform the processes described herein. For example, the processesdescribed herein may also be implemented as a non-transitorymachine-readable storage medium, configured with a computer program,where upon execution, instructions in the computer program cause thecomputer to operate in accordance with the processes. A non-transitorymachine-readable medium may include but is not limited to a hard drive,compact disc, flash memory, non-volatile memory, volatile memory,magnetic diskette and so forth but does not include a transitory signalper se.

The processes described herein are not limited to the specific examplesdescribed. For example, the processes 400 and 500 are not limited to thespecific processing order of FIGS. 4 and 5, respectively. Rather, any ofthe processing blocks of FIGS. 4 and 5 may be re-ordered, combined orremoved, performed in parallel or in serial, as necessary, to achievethe results set forth above.

The processing blocks (for example, in the processes 400 and 500)associated with implementing the system may be performed by one or moreprogrammable processors executing one or more computer programs toperform the functions of the system. All or part of the system may beimplemented as, special purpose logic circuitry (e.g., an FPGA(field-programmable gate array) and/or an ASIC (application-specificintegrated circuit)). All or part of the system may be implemented usingelectronic hardware circuitry that include electronic devices such as,for example, at least one of a processor, a memory, a programmable logicdevice or a logic gate.

Elements of different embodiments described herein may be combined toform other embodiments not specifically set forth above. Otherembodiments not specifically described herein are also within the scopeof the following claims.

What is claimed is:
 1. A method comprising: checking periodically, usinga processor, for a value associated with data in a data block in ahigher tier journal stored on a first storage array havingdeduplication-based functionality, the higher tier journal comprising: aDO stream comprising new data to be written to the first storage array;a DO METADATA stream comprising a pointer to an offset in the DO streamwhere the corresponding new data is located; an UNDO stream comprisingold data overwritten in the first storage array; and an UNDO METADATAstream comprising a pointer to an offset in the UNDO stream where thecorresponding old data is located; copying the data in the data blockfrom the higher tier journal to a lower tier journal in a second storagearray in response to a determination that the data in the data blockexists only in the UNDO stream in the higher tier journal, the firststorage array and the second storage array being used in a continuousdata protection system to replicate a volume; replacing the data in thedata block in the higher tier journal with a zero block in response to adetermination that the data in the data block exists only in the UNDOstream; and marking a block in metadata in the UNDO METADATA stream,corresponding to the data block and pointing to the offset in the UNDOstream of the higher tier journal, to point to an offset of an UNDOstream in the lower tier journal.
 2. The method of claim 1, whereinchecking periodically for the value associated with data in the datablock in the journal stored on the first storage array havingdeduplication-based functionality comprises checking periodically forthe value associated with data in the data block in the journal storedon a first storage array having flash-based functionality.
 3. The methodof claim 1, wherein checking periodically for the value compriseschecking periodically for a reference count.
 4. The method of claim 1,wherein copying the data in the data block from the higher tier journalto the lower tier journal comprises copying the data in the data blockfrom the higher tier journal to the lower tier journal if one of: thedata is older than a predetermined amount of time, or the data in theUNDO stream consumes more than a predetermined amount of data.
 5. Themethod of claim 1, further comprising returning data in a replicatedstorage of a user volume to an earlier point in time, returning the datacomprising: reading data from the lower tier journal if the block in themetadata in the UNDO METADATA stream in the higher tier journal pointsto the offset of the UNDO stream in the lower tier journal; copying datafrom the replicated volume to the DO stream; updating the DO METADATAstream in the higher tier journal; and applying data from the UNDOstream of the lower tier journal and the UNDO stream of the higher tierjournal to the replicated volume.
 6. The method of claim 5, whereinapplying data from the UNDO stream of the higher tier journal to thereplicated volume comprises applying the data using one of an xcopycommand, a read signature command or a write signature command.
 7. Anapparatus, comprising: electronic hardware circuitry configured to:check periodically for a value associated with data in a data block in ahigher tier journal stored on a first storage array havingdeduplication-based functionality, the higher tier journal comprising: aDO stream comprising new data to be written to the first storage array;a DO METADATA stream comprising a pointer to an offset in the DO streamwhere the corresponding new data is located; an UNDO stream comprisingold data overwritten in the first storage array; and an UNDO METADATAstream comprising a pointer to an offset in the UNDO stream where thecorresponding old data is located; copy the data in the data block fromthe higher tier journal to a lower tier journal in a second storagearray in response to a determination that the data in the data blockexists only in an UNDO stream in the higher tier journal; replace thedata in the data block in the higher tier journal with a zero block inresponse to a determination that the data in the data block exists onlyin the UNDO stream; and mark a block in metadata in the UNDO METADATAstream, corresponding to the data block and pointing to the offset inthe UNDO stream of the higher tier journal, to point to an offset of anUNDO stream in the lower tier journal, wherein the first storage arrayand the second storage array are used in a continuous data protectionsystem to replicate a volume.
 8. The apparatus of claim 7, wherein thecircuitry comprises at least one of a processor, a memory, aprogrammable logic device or a logic gate.
 9. The apparatus of claim 7,wherein the circuitry configured to check periodically for the valueassociated with data in the data block in the journal stored on thefirst storage array having deduplication-based functionality comprisescircuitry configured to check periodically for the value associated withdata in the data block in the journal stored on a first storage arrayhaving flash-based functionality.
 10. The apparatus of claim 7, whereinthe circuitry configured to check periodically for the value comprisescircuitry configured to check periodically for a reference count. 11.The apparatus of claim 7, wherein the circuitry configured to copy thedata in the data block from the higher tier journal to the lower tierjournal comprises circuitry configured to copy the data in the datablock from the higher tier journal to the lower tier journal if one of:the data is older than a predetermined amount of time, or the data inthe UNDO stream consumes more than a predetermined amount of data. 12.The apparatus of claim 7, further comprising circuitry configured toreturn data in a replicated storage of a user volume to an earlier pointin time, the circuitry configured to return the data comprisingcircuitry configured to: read data from the lower tier journal if theblock in the metadata in the UNDO METADATA stream in the higher tierjournal points to the offset of the UNDO stream in the lower tierjournal; copy data from the replicated volume to the DO stream; updatethe DO METADATA stream in the higher tier journal; and apply data fromthe UNDO stream of the lower tier journal and the UNDO stream of thehigher tier journal to the replicated volume.
 13. The apparatus of claim12, wherein the circuitry configured to apply data from the UNDO streamof the higher tier journal to the replicated volume comprises circuitryconfigured to apply the data using one of an xcopy command, a readsignature command or a write signature command.
 14. An articlecomprising: a non-transitory computer-readable medium that storescomputer-executable instructions, the instructions causing a machine to:check periodically for a value associated with data in a data block in ahigher tier journal stored on a first storage array havingdeduplication-based functionality, the higher tier journal comprising: aDO stream comprising new data to be written to the first storage array;a DO METADATA stream comprising a pointer to an offset in the DO streamwhere the corresponding new data is located; an UNDO stream comprisingold data overwritten in the first storage array; and an UNDO METADATAstream comprising a pointer to an offset in the UNDO stream where thecorresponding old data is located; copy the data in the data block fromthe higher tier journal to a lower tier journal in a second storagearray in response to a determination that the data in the data blockexists only in an UNDO stream in the higher tier journal; replace thedata in the data block in the higher tier journal with a zero block inresponse to a determination that the data in the data block exists onlyin the UNDO stream; and mark a block in metadata in the UNDO METADATAstream, corresponding to the data block and pointing to the offset inthe UNDO stream of the higher tier journal, to point to an offset of anUNDO stream in the lower tier journal, wherein the first storage arrayand the second storage array are used in a continuous data protectionsystem to replicate a volume.
 15. The article of claim 14, wherein theinstructions causing the machine to check periodically for the valueassociated with data in the data block in the journal stored on thefirst storage array having deduplication-based functionality comprisesinstructions causing the machine to check periodically for the valueassociated with data in the data block in the journal stored on a firststorage array having flash-based functionality.
 16. The article of claim14, wherein the instructions causing the machine to check periodicallyfor the value comprises instructions causing the machine to checkperiodically for a reference count.
 17. The article of claim 14, whereinthe instructions causing the machine to copy the data in the data blockfrom the higher tier journal to the lower tier journal comprisesinstructions causing the machine to copy the data in the data block fromthe higher tier journal to the lower tier journal if one of: the data isolder than a predetermined amount of time, or the data in the UNDOstream consumes more than a predetermined amount of data.
 18. Thearticle of claim 14, further comprising instructions causing the machineto return data in a replicated storage of a user volume to an earlierpoint in time, the instructions causing the machine to return the datacomprising instructions causing the machine to: read data from the lowertier journal if the block in the metadata in the UNDO METADATA stream inthe higher tier journal points to the offset of the UNDO stream in thelower tier journal; copy data from the replicated volume to the DOstream; update the DO METADATA stream in the higher tier journal; andapply data from the UNDO stream of the lower tier journal and the UNDOstream of the higher tier journal to the replicated volume.
 19. Thearticle of claim 18, wherein the instructions causing the machine toapply data from the UNDO stream of the higher tier journal to thereplicated volume comprises instructions causing the machine to applythe data using one of an xcopy command, a read signature command or awrite signature command.
 20. The method of claim 1, replacing the datain the data block in the higher tier journal with a zero block if thedata in the data block exists only in the UNDO stream comprisesreplacing the data in the data block in the higher tier journal with azero block by using SCSI (Small Computer System Interface) punchcommand.